MODEL expects a continuous CV, not a trigger (or a gate). With that input CV, you select the synthesis model: the higher the voltage you feed it, the higher up the two banks of models you go.
A trigger (or gate) is a very simple CV that typically can only have two values: 0V and “high” (5V, or 8V, or 10V, depending on the module). That’s why it can only select two models: at 0V, it selects the model you selected manually using the buttons on Plaits; at its high voltage (when it triggers) it selects the last model in the range, the hi-hats. If you want to select other models, you need a CV that can do everything between 0V and “high”!
Instead of feeding that trigger to MODEL, try feeding it to TRIG, and send the Y output (or one of the X outputs) from Marbles to MODEL. Then, every time you trigger the TRIG input, Plaits plays the synthesis model currently selected by the CV going into MODEL.